The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal display device operating in a so-called VA-(vertically aligned) mode in which liquid crystal molecules are aligned generally perpendicularly to a liquid crystal layer.
Liquid crystal display devices are used extensively for a display device of various information processing apparatuses such as a computer. Liquid crystal display devices, having a compact size and consuming little electric power, are particularly suitable for application in portable information processing apparatuses. On the other hand, use of such liquid crystal display devices also in a fixed-type information processing apparatus such as a desktop-type computer, is also studied intensively.
Conventional liquid crystal display devices generally use a so-called TN (Twisted Nematic)-mode construction in which p-type liquid crystal molecules having a positive dielectric anisotropy are aligned horizontally between a pair of mutually opposing panel substrates, such that the liquid crystal molecules adjacent to one panel substrate and the liquid crystal molecules adjacent to the other panel substrate are aligned in respective directions crossing with each other perpendicularly.
In such a TN-mode liquid crystal display device, a pair of polarizers are disposed above and below the liquid crystal panel with a crossed Nicol state in which respective optical absorption axes intersect each other perpendicularly. Thereby, a white representation is given in a non-activated state of the liquid crystal display device in which no driving electric field is applied across the liquid crystal layer. In an activated state in which a driving electric field is applied across the liquid crystal layer, on the other hand, a black representation is obtained.
In the foregoing black representation mode of the TN-mode liquid crystal display device, it should be noted that the liquid crystal molecules are aligned generally vertically to the principal surface of the panel substrate as a result of the driving electric field thus applied across the liquid crystal layer, and the transmission of the incident optical beam through the liquid crystal display device is effectively interrupted by the polarizers disposed above and below the liquid crystal panel.
In such a black representation mode, however, the liquid crystal molecules immediately adjacent to the panel substrate tend to maintain the horizontal alignment even when the driving electric field is applied. Thereby, the polarization state of the incident optical beam is affected by such horizontally aligned liquid crystal molecules and leakage of light occurs even in the black representation mode. In other words, conventional TN-mode liquid crystal display devices have suffered from the problem of low contrast ratio of representation.
A VA-mode liquid crystal display device is a liquid crystal display device in which liquid crystal molecules having a negative dielectric anisotropy are confined between a pair of panel substrates in a state that the liquid crystal molecules are aligned in a direction generally perpendicular to the principal surface of the liquid crystal layer in the non-activated state of the liquid crystal display device. Thus, an incident optical beam passes through the liquid crystal layer without changing the polarization plane thereof in the non-activated state of the liquid crystal device, and the incident optical beam is effectively interrupted by a pair of polarizers disposed above and below the liquid crystal layer in a crossed Nicol state. In such a VA-mode liquid crystal display device, therefore, it is possible to achieve a near-ideal black representation in the non-activated state of the liquid crystal display device. In other words, such a VA-mode liquid crystal display device can easily achieve a very high contrast representation not achievable by a conventional TN-mode liquid crystal display device.
When a VA-mode liquid crystal display device is to be used for a desktop information processing apparatus, the VA-mode liquid crystal display device is required, in addition to the high contrast ratio, to have a wide view angle and a high response speed. In relation to the view angle, conventional VA-mode liquid crystal display devices have generally suffered from the problem of inverted gradation which occurs when the liquid crystal panel is viewed from a direction in which the liquid crystal molecules cause a tilting upon application of the driving electric field.
In conventional TN-mode liquid crystal display devices, such a problem of view angle has been dealt with by dividing each pixel into a plurality of domains having respective alignment directions of the liquid crystal molecules. The art of forming such a so-called divided alignment domain structure is well established in the art of TN-mode liquid crystal display devices.
FIGS. 1A and 1B show respectively the surface of a conventional TN-mode liquid crystal display device and a corresponding cross-sectional structure in an activated state thereof.
Referring to the cross sectional view of FIG. 1B, the liquid crystal display device includes a liquid crystal layer 2 sandwiched between a substrate 1A and a substrate 1B, and the liquid crystal layer 2 is divided into a domain A and a domain B adjacent to the domain A, wherein it should be noted that the direction of tilting of the liquid crystal molecules, which occurs in response to the application of a driving electric field to the liquid crystal layer 2, is opposite in the domain A and in the domain B. Further, it should be noted that, at the boundary between the domain A and the domain B, there is formed a boundary region in which the liquid crystal molecules maintain a horizontal alignment even when a driving electric field is applied thereto. In correspondence to the boundary region thus formed, a bright disclination line is observed as indicated in FIG. 1A. In FIG. 1A, it should be noted that the polarizers above and below the liquid crystal panel are disposed in the crossed Nicol state and the domains A and B can be seen as black bands.
Thus, one may expand the use of the foregoing teaching of divided alignment domain structure further to the case of a VA-mode liquid crystal display device for improving the view angle characteristics thereof.
FIG. 2 shows a simple expansion of the divided alignment domain structure to a VA-mode liquid crystal display device as noted above, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 2, the liquid crystal layer 2, which now includes vertically aligned liquid crystal molecules, is divided into a domain A and an adjacent domain B, wherein the direction of tilting of the liquid crystal molecules is opposite in the domain A and in the domain B. However, there is little investigation made on such a divided alignment domain structure in a VA-mode liquid crystal display device, and it is not clear at all whether the structure of FIG. 2 can be actually formed at all as a stable structure. It should be noted that the structure of FIG. 2 is merely a hypothetical structure and the stability or feasibility thereof is not yet investigated.